Streptonigrin, 5'-amino-6'-(7-amino-5,-8-dihydro-6-methoxy-5,8-dioxo-2-quinolyl)-4'-(8'-h ydroxy-9',10'-dimethoxyphenyl)-3'-methyl-2'-pyridinecarboxylic acid, having the structure: ##STR1## has previously been obtained from the microorganism Streptomyces flocculus. While streptonigrin has shown activity as an antitumor antibiotic, this compound also exerts a significant toxic effect, for example, in bone marrow depression. The art has thus continued to seek less toxic, as well as more potent, antitumor antibiotic agents.